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Proper Voltage at Alternator Field Control Terminals?

EMSJEEP

NAXJA Forum User
Location
Long Island
Should they both be 12v+? Is one low voltage or ground? 2001 XJ

I'm putting out 4v from the alternator at idle, which is killing the battery.

Thanks
 
I have 12.5v at one (I have significant voltage drop across my battery isolator) but the other does not appear to be ground. I think I'll run an independent ground direct to the battery and see if that helps. I.e. If it's supposed to be a ground, then it's blown and hopefully that's my problem. The nuts on the terminals had worked themselves almost completely off when I went to check, cleaning and tightening didn't help though...
 
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Voltage *in* is constant voltage, typically (near) battery voltage, dark green/orange wire.

The alternator regulates on the ground side, dark green wire.

Typically a voltage drop on the green wire, depends on how it is regulating at the moment.


Grounding the green wire can smoke your alternator.
 
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Voltage *in* is constant dark green/orange wire.

The alternator regulates on the ground side, dark green wire.

Typically a voltage drop on the green wire, depends on how it is regulating at the moment.


Grounding the green wire can smoke your alternator.

K, so won't do that....

So when I put red probe on right and black probe on left, I get nothing, red probe right and black probe to bony gets 12.5v, red probe to left and black probe to body gets either 0 or a low voltage (like 3v)
 
I haven't worked on a 2001 specifically, but the basics seem to be the same. I use the wire colors instead of left or tight or the pole number.

On mine 96 I get battery voltage on the green orange and around 9 volts on the green, The green side depends on whether the battery is (near) full and if it is right after startup or after the motor warms up.

If you are getting battery voltage to the orange/green wire to ground and have a reasonable voltage drop from the green wire to ground, you are likely regulating. If the voltage is the same it is likely either an open wire or the PCM isn't regulating.
 
I have nothing or almost nothing on one terminal and a constant 4v out to the battery from the large lug.
 
Sounds like you have an open field winding. Take the wires off and ohm between the two field winding posts on the alternator.

General Tip, double check all readings, try different ground points. Oil, rust, dirt or whatever can mess up your readings, clean shiny metal or strat to battery negative are the safe points (usually).
 
I have nothing or almost nothing on one terminal and a constant 4v out to the battery from the large lug.

Reading that large lug doesn't tell you much, it can either be the alternator output or the battery voltage. It isn't a one way wire.

Try an ohm test on one or both of the field wires to the case and see if there is a short.

Could be your alternator, could be our PCM/voltage regulator, could be the wiring. Guessing gets real expensive real quick.

Kind of sounds like you have an open circuit in your field winding (just a guess). Which could be a bad alternator or the connection at the plug. I'm not sure of the setup on the 2001, but is it possible somebody over tightened the nuts onto the field winding poles and broke studs off inside the alternator?
 
I do not know the wire colors on the 2001 XJ, but this is how it work on the 1996 XJ:

Dark Green and Orange (DG/OR) from the ASD (Automatic Shut Down) relay and provides 12 volts to the alternator when the ignition is on.

Dark Green (DG) is the field driver. It provides a negative to the alternator field wire to control the voltage coming from the alternator to the system. The field driver is PCM controlled. The PCM pulses the negative on and off in an attempt to keep the voltage level within specs.

When testing the alternator in the XJ, I attach a wire to this terminal and with the engine running temporarily touch the other end of the wire to a clean ground and monitor the voltage at the battery. With a good alternator, the voltage could quickly climb past 16 volts.
 
Reading that large lug doesn't tell you much, it can either be the alternator output or the battery voltage. It isn't a one way wire.

Try an ohm test on one or both of the field wires to the case and see if there is a short.

Could be your alternator, could be our PCM/voltage regulator, could be the wiring. Guessing gets real expensive real quick.

Kind of sounds like you have an open circuit in your field winding (just a guess). Which could be a bad alternator or the connection at the plug. I'm not sure of the setup on the 2001, but is it possible somebody over tightened the nuts onto the field winding poles and broke studs off inside the alternator?

Mine is on a diode for the battery isolator
 
I do not know the wire colors on the 2001 XJ, but this is how it work on the 1996 XJ:

Dark Green and Orange (DG/OR) from the ASD (Automatic Shut Down) relay and provides 12 volts to the alternator when the ignition is on.

Dark Green (DG) is the field driver. It provides a negative to the alternator field wire to control the voltage coming from the alternator to the system. The field driver is PCM controlled. The PCM pulses the negative on and off in an attempt to keep the voltage level within specs.

When testing the alternator in the XJ, I attach a wire to this terminal and with the engine running temporarily touch the other end of the wire to a clean ground and monitor the voltage at the battery. With a good alternator, the voltage could quickly climb past 16 volts.

If the green wire is pulse your meter may not read it, some do, some don't, depends on the frequency of the pulse. If you have an analog meter you may get different results. Just a thought. I have two VOMs and a couple of old analog meters, when I get a really funky reading I swap out meters. Make sure you are trying to read the DC position and not the AC position on your meter. And just for giggles switch over to the AC node and see what you read, bad diodes can cause some odd readings.
 
Do you have dual batteries, why the isolator?

Yeah, house batteries in the trunk for HAM radio, emergency lighting and compressor on a 5 gal tank
 
Yeah, house batteries in the trunk for HAM radio, emergency lighting and compressor on a 5 gal tank

Isolator is usually wired between batteries and not between the alternator and the batteries? Though with a 4 volt output on your main cable this is unlikely to be the issue IMO.

I don't really know the setup for a 2001, but on mine the alternator main power out cable goes to a junction on the side of the PDC and then jumps to battery positive. A side note, if yours does have the junction, check the nut for tightness.

Do you have a stock Denso alternator? That four volts on the main power out from the alternator sure sounds like a generator reading, if in fact that cable is isolated. But don't let me confuse the issue, just thinking out loud. The residual magnetism in the field winding magnets will give you four volts if the field current is interrupted, this is only on a true generator with field magnets, not an alternator.

How are you doing your tests, the engine has to be running. And how does it run on four volts? Something isn't adding up here or I'm confusing myself. :)
 
My bad, I looked up how the newer isolators are wired, they have a diode on both battery legs.

Best guess is an open or shorted field winding. Second best guess is a faulty regulator or wiring. A faulty rectifier is unlikely but possible, one way to test for this is to switch your meter over to AC mode, some meters will pick up a failed diode in the rectifier.

Your field winding may be open where you mentioned the nuts being loose on the alternator connector. Is it possible you spun one of the field winding pole studs and messed up the connection, just a wild guess?
 
Isolator is usually wired between batteries and not between the alternator and the batteries? Though with a 4 volt output on your main cable this is unlikely to be the issue IMO.

I don't really know the setup for a 2001, but on mine the alternator main power out cable goes to a junction on the side of the PDC and then jumps to battery positive. A side note, if yours does have the junction, check the nut for tightness.

Do you have a stock Denso alternator? That four volts on the main power out from the alternator sure sounds like a generator reading, if in fact that cable is isolated. But don't let me confuse the issue, just thinking out loud. The residual magnetism in the field winding magnets will give you four volts if the field current is interrupted, this is only on a true generator with field magnets, not an alternator.

How are you doing your tests, the engine has to be running. And how does it run on four volts? Something isn't adding up here or I'm confusing myself. :)

Its a Powermaster 170 amp alternator. I ran it fine for about 6 years on the setup with the isolator. The Jeep sat without running for a year and I'm coming off of that right now.

In total it has new mains as well to run the upgraded alternator and accessories .

The isolator has an "Alternator In" Lug, and a "Battery 1 Out" and a "Battery 2" out lug. Between the Alternator lug and the Alternator sits a 200amp circuit breaker. Mine does not go to the PDC, but there is a jumper from the PDC to Battery 1 (under the hood) which supplies vehicle power.

I got the Jeep running again last week after a new CPS sensor, fuel pump and a bunch of other housekeeping after being not turned on for over a year. It has been out of regular service since 2012.

Once I got it running last week I let it run for a few hours without difficulty, then left it for a week, when I came back, it started up again just fine and I left it to idle for a few hours. When I got back, it hadn't burned much fuel and the battery was dead, so I think it cut off within an hour or so of when I started it to let it run. I recharged the battery off of an AC charger and started it again, let it run, it died a few minutes later. I had bought a new Odyssey battery anyway, so I threw that in thinking that the 3 yr old battery that has sat for a while was maybe not good enough to run off of. So I put the new Odyssey in and it started fine and ran. I started checking with a DMM to see what was up and noticed that at the lug on the back f the alternator and grounded to battery negative, I was only getting 4 VDC and that the battery voltage was steadily dropping from 13.0VDC to 12.4VDC.

So, I've been running the Jeep a few minutes at a time to check things out, but not dipping below 12.3 on the new battery, and charging it up with the 6 amp AC trickle charger between attempts.
 
My bad, I looked up how the newer isolators are wired, they have a diode on both battery legs.

Best guess is an open or shorted field winding. Second best guess is a faulty regulator or wiring. A faulty rectifier is unlikely but possible, one way to test for this is to switch your meter over to AC mode, some meters will pick up a failed diode in the rectifier.

Your field winding may be open where you mentioned the nuts being loose on the alternator connector. Is it possible you spun one of the field winding pole studs and messed up the connection, just a wild guess?

When I noticed things weren't working, I checked the terminals and the nuts were hanging near the ends of their threads and the rings barley making contact. I hadn't touched them in years before it stopped working. I don't think they are close enough to cross each other, but it's possible. I tightened them down but didn't really wrench on them.
 
Just went out again. Steady 12VDC to one terminal and the other is jumping all over on my meter, but a steady ~3v at the battery out lug.
 
Just for the heck of it, switch your meter to AC volts and check that alternator to isolator circuit again. A faulty rectifier may show up as AC voltage on some meters.

Do the ohm test on your field windings,

Double check your voltage on the regulated side of the field winding circuit to a good ground (with the engine running), pretty sure it is the solid green wire. If it is the same voltage as the in wire (orange/green) you likely have a regulator issue. If there is no voltage you likely have a winding or no power in issue.

Check the voltage at the field winding in wire orange/green with the engine running.

If things get too crazy you may have to pull the alternator and take it in for a bench test. A real pain. If it is the regulator you may have pulled the alternator for nothing. I don't envy you all the crawling around.
 
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